There have been numerous examples of instruments and methods for detecting and measuring specific gases present in an atmosphere. For example, microcalorimetric gas sensors, (pellistors) burn combustible gases with the surrounding air on the surface of a small ball or film of catalytically active metal. The catalyst, e.g. Pt, Pd, or Rh is kept at 500-600° C. The heat of combustion in the presence of a gas is balanced by a reduction in electrical heating power. The power consumption serves as the signal indicating a concentration of flammable gases. This type of sensor is the current standard for the detection of explosives in plants, because it shows a higher accuracy and longer-term stability than the (cheaper) oxidic extensor prevailing in-home applications for the same purpose. Examples include those shown in Debeda, H, Rebiere D, Pistre J, and Menil J 1995 Sensors Actuators B 27 297-300.
Electrochemical gas cells ionize the gas molecule at a three phase boundary layer (atmosphere, electrode of a catalytically active material, electrolyte). Some examples of electrode materials are platinum for CO, gold for a NO2, and activated coal for SO2 detection. Examples of these cells are shown in Brailsford A D, Yussougg M and Logothetis E M 1992 Technical Digest of the 4th Meeting of Chemical Sensors (Tokyo) ed N Yamazoe (Japan Association of Chemical Sensors) p 160.
Mass sensitive piezoelectric sensors detect a weight change of an absorbtive layer by use of a quartz microbalance or a surface acoustic wave substrate. Examples of these devices are described in Lucklum R, Hauptmann P 2000 Sensors Actuators B 70 30-6.
Field effect transistors (FET) have also been used as gas sensing devices. Typically, in these arrangements, the gate metal is exposed to the surrounding atmosphere and hydrogen or hydrogen containing gases disassociate or decompose on the surface and the protons defuse to the metal/insulator interface and influence the charge in the semiconductor, thereby changing the drain source current. Examples of such arrangements include those described in Tobias P, Martensson P, Baranzahi A, Solomonsson P, and Lundstrom I 1998 Sensors Actuators B 47 125-30 and Lampe U, Gerblinger J and Meixner H, 1992 Sensors Actuators B 7 787-94.
A crucial aspect of the preparation of gas sensors is the deposition of the sensing layer on a substrate surface. Known methods for the deposition of this sensing layer include paste/slurry deposition, chemical vapor deposition (CVD), and physical vapor deposition (PVD). The various chemical and physical vapor deposition (CVD or PVD) techniques are mostly standard processes in the semiconductor industry, the liquid deposition techniques are less frequently employed. However, the compatibility of the latter, i.e. screenprinting and drop deposition techniques with semiconductor processes have been shown to be feasible.
One example of a gas sensor is shown in U.S. Pat. No. 5,470,756 issued to Coles et al. Nov. 28, 1995. All documents, patents, journal articles, and other materials cited in the present application are hereby incorporated by reference.
As described by Coles, a gas sensitive layer is formed of SnO2 incorporating BiO3 in an amount less than 35%, but sufficient to confer hydrogen sensitivity and selectivity. Coles further contemplates the inclusion of the catalyst selected from the group of metals Ir, Pt, Ag, Ru, Au or Pd. Coles teaches the deposition of these materials on a substrate as a slurry.
Drawbacks of the prior art methods include slow response times, low sensitivity, high manufacturing costs, and difficulty in reproducing consistent results. Accordingly, new materials and methods of fabrication are needed to improve gas sensors. The present invention is directed towards such materials and methods.